Regulations to limit the amount of volatile organic compounds (VOC) of industrial coatings has encouraged research and development projects directed at inventing new waterborne systems such as paints. With respect to the two most important commercial coating systems, polyesters and acrylics, it is a relatively easy task to characterize the current state-of-the-art: polymeric systems with a carbon backbone (acrylics) are more stable to hydrolysis than polyesters with their carbon-oxygen ester backbone chain. However, considerable research continues toward improving the stability of aqueous polyesters because of their inherently desirable properties of excellent hardness/flexibility ratios and outstanding gloss unattainable from aqueous acrylic systems.
U.S. Pat. No. 4,340,519 discloses the composition of certain crystalline and non-crystalline polyesters copolymerized with a metal sulfonate group-containing aromatic acid and up to 10 mol percent (based on the total polyol content) of a polyvalent polyol selected from trimethylolpropane, trimethylolethane, glycerine, and pentaerythritol. Also, U.S. Pat. No. 4,525,524 discloses liquid systems comprised of polyesters containing certain metal sulfonates and, optionally, up to 3 percent of a branching agent based upon the total diol component.
U.S. Pat. No. 3,563,942 discloses linear solvent-soluble copolyester compositions that can be dispersed in water. Water dispersibility is gained by the addition to the copolyester of a small amount (1-2 mol percent) of the metal salt of sulfonated aromatic compounds.
Many patents disclose methods to obtain water-dissipatable polyesters by neutralizing residual or unreacted carboxylic acid groups on the polymer with ammonia or various organic amines. U.S. Pat. No. 3,666,698 utilizes this method as well as phenylindandicarboxylic acid to modify coating performance. U.S. Pat. No. 3,699,066 shows the benefits of certain hydroxy-functional amines for neutralization. U.S. Pat. No. 3,549,577 utilizes the aminoresin crosslinker as the neutralizing agent then adjusts the pH to prepare an alkaline water-reduced system. In these patents as well as U.S. Pat. Nos. 3,494,882, 3,434,987, U.K. 1,117,126, and U.S. Pat. No. 3,345,313 carboxylic acid functionality is completely neutralized with excess base yielding an alkaline paint vehicle.
Amine neutralized alkyd or oil-free polyester resin used in waterborne coatings is cured at ambient temperature or by baking. In these resins, the carboxyl end-groups are neutralized with an amine such as ammonium hydroxide, triethyl amine, or N,N'-dimethylaminoethanol (DMAE) to yield ammonium salts which can then be dispersed in water to give waterborne enamels. When the enamel is cured, the amine is given off.
U.S. Pat. Nos. 4,910,292, 4,973,656, and 4,990,593 the disclosures of which are incorporated herein in their entirety by reference, disclose an improved novel water-borne polyester resin and coatings specifically containing 5-(sodiosulfo)-isophthalic acid (5-SSIPA or SIP). The resins are prepared by reacting neopentyl glycol (NPG), trimethylolpropane (TMP), isophthalic acid (IPA), and 5-SSIPA in the first stage, followed by the addition of IPA and adipic acid (AD) in the second stage. The clear coating prepared from this resin exhibits good properties of gloss, hardness, flexibility, and solvent resistance. Moreover, the coating is water-resistant, despite the presence of ionic sulfonate groups. However, as with other polyester resins, aqueous dispersions of these resins do not have adequate storage stability and phase-separate over time. Additionally, the pigmented enamels prepared from these dispersions do not have adequate storage stability and phase separate in a relatively short period of time.
A solution to the stability problem is disclosed in U.S. application Ser. No. 07/765,606 filed Sep. 25, 1991, the disclosure of which is incorporated herein in its entirety by reference. This application discloses end-capping the dicarboxylic acid sulfomonomer. However, pigmented coatings of these compositions are not very water resistant and form wrinkles when cured.
It would, therefore, be very desirable to be able to improve the overall stability, coating smoothness, and coating water resistance of pigmented enamels prepared from dispersions of water-dispersible polyester resins.